An arrival direction of radio wave is conventionally estimated accurately in a method such as Multiple Signal Classification (MUSIC) method, using an array antenna comprising a plurality of antenna elements. The MUSIC method is disposed in R. O. Schmidt, “Multiple Emitter Location and Signal Parameter Estimation”, Institute of Electrical and Electronics Engineers (IEEE) Trans., AP-34, pp. 276-280 (1986). This method includes an algorism for accurately estimating a direction of a plurality of incident waves with the same frequency band.
In this method, M (>1) antenna elements receive signals, and a receiving unit connected to each antenna element converts the frequency of each of the received signals, detects a phase of it, and demodulates the received signal to a signal comprising orthogonal I and Q signals. An analog/digital converter (A/D converter) converts the demodulated signal to sampling data and outputs the data to a direction estimating processor. The direction estimating processor estimates a direction of the incident waves using the sampling data by the MUSIC method. In other words, using sampling data x1(k), x2(k), . . . , xM(k) at sampling time kΔT obtained by respective antenna elements, a correlation matrix calculation unit creates receiving vector x (k) written asx(k)=[x1(k)x2(k) . . . xM(k)]  (Equation 1),where T shows transposition of a vector, ΔT is a sampling interval, and k is a natural number. The correlation matrix calculation unit, using receiving vectors x (k) for k=1 to N, further finds M×M correlation matrix R written as                               R          =                                    ∑                              k                =                1                            N                        ⁢                                                   ⁢                                          x                ⁡                                  (                  k                  )                                            ⁢                                                                    x                    ⁡                                          (                      k                      )                                                        H                                /                N                                                    ,                            (                  Equation          ⁢                                           ⁢          2                )            where H shows complex conjugate transposition of a vector.
The calculation unit calculates eigenvalues λ1-λM of correlation matrix R in the descending order, and eigenvactors e1-eM corresponding to eigenvalues λ1-λM.
Next, the calculation unit calculates an evaluation value of an arrival-angle evaluation function, assuming number of the incident waves is S, and using noise spatial eigenmatrix EN=[eS+1, . . . , eM] and a feature that signal eigenvector space ES=[e1, . . . , eS] and EN are orthogonal to each other. This EN is formed with (M−S) eigenvactors, namely column vectors, belonging to a noise eigenvactor space having the relation written asλ1≧λ2≧ . . . ≧λs>λs+1=λs+2= . . . =λM  (Equation 3),and ES is formed with eigenvactors e1-eS. In other words, arrival-angle evaluation function F(θ) for evaluating orthogonality between EN and ES is defined by                                           F            ⁡                          (              θ              )                                =                      1                                                            a                  H                                ⁡                                  (                  θ                  )                                            ⁢                              E                N                            ⁢                              E                N                H                            ⁢                              a                ⁡                                  (                  θ                  )                                                                    ,                            (                  Equation          ⁢                                           ⁢          4                )            where a(θ) is a complex response (hereinafter called a steering, vector) of the array antenna as a function of azimuth θ. Azimuth θ varies in a predetermined angle range. When azimuth θ equals to the arrival angle, ideally, arrival-angle evaluation function F(θ) is infinite. A resultant peak direction of F(θ) from calculation for the varied θ is set to be the arrival-angle evaluation value of the incident waves.
Number S of incident waves is generally unknown, so that the number is determined based on an eigenvalue distribution and number-of-signal determination criteria. The criteria is described in M. Wax and T. Kailath, “Detection of Signals by Information Theoretic Criteria”, IEEE Trans. On Acoustics, Speech and Signal, Processing, Vol. ASSP 33 (2), pp. 387-392, February (1985).
The radio-wave arrival-direction estimating apparatus employing the MUSIC method discussed above estimates an arrival direction accurately by signal processing, using an algorithm of deriving eigenvalue of a correlation matrix of array received signals. In such an estimating apparatus, correlation between waves generated by reflection on the ground or a building increases when a relative delay time between these waves is shorter than a symbol length. In this case, correlation matrix R degrades, and therefore the incident waves cannot be precisely separated.
For preventing the degradation, a spatial smoothing technique is proposed. This spatial smoothing technique is described in Pillai et al, “Forward/Backward Spatial Smoothing Techniques for Coherent Signal Identification”, IEEE Trans. On Acoustics, Speech and Signal Processing, Vol. 37, No. 1, 1989. The example has estimated the arrival direction using spatial samples from the array antenna; however the MUSIC method can be similarly applied to a signal sampled every frequency and the delay time of the received waves can be estimated at high resolution.
The estimation accuracy of the arrival direction in the MUSIC method depends on variation step Δθ of θ in the arrival-angle evaluation function (Eq.4). When Δθ increases, a calculation amount in the entire variation range of θ decreases, but the peak direction of the arrival-angle evaluation function cannot accurately detect the peak direction and the accuracy decreases. When Δθ decreases, the peak direction of the arrival-angle evaluation function can be accurately detected, but a calculation amount in the entire variation range of θ increases.